Code no longer builds serially

[guadabsb15]

This comment is also part of the review openjournals/joss-reviews#6637

With the changes in commit 54875fa, the preprocessor directives for #ifdef NUDUSTC_ENABLE_MPI were commented out in src/main.cpp at lines 21 and 23, 60 and 65. Therefore, this code sections are being compiled even when building serially.

If these lines are uncommented, the code can be built again by running cmake -DNUDUSTC_ENABLE_MPI=OFF .. and make

[steven-murray]

Ah, this is why my compilation fails :-)